The present invention relates to a fuel supply system for an internal combustion engine, and more particularly to a single point injection type fuel supply system in which a single fuel injection valve is disposed in the area where a plurality of branch passages of an intake manifold join.
In a conventional method, for all running conditions of the engine, the amount of fuel supplied is controlled by varying the pulse width of a signal controlling the electronic fuel injection valve.
However, according to the conventional method, the area of the opening of the fuel injection valve is set such that when the width of the driving pulse corresponding to a valve opening time of the fuel injection valve is maximum, that is, when the valve is substantially held continuously fully opened, the maximum amount of fuel required is supplied. Accordingly, in the range of low air intake conditions, particularly in the extreme case when the engine is idling, an extremely narrow driving pulse corresponding to the valve opening time is required.
In a usual gasoline internal combustion engine for an automotive vehicle, it is required that the maximum amount of fuel supply required per unit of time be approximately forty times or more as large as the amount of fuel supplied at the time of idling. In other words, it is required that the width of the driving pulse corresponding to the valve opening time of the fuel injection valve at the time of idling be approximately one fortieth or less of that at the time when the fuel injection valve is continuously held fully opened.
For instance, in an engine which idles at 600 r.p.m., the time required for each revolution is 0.1 sec, that is, 100 msec. Accordingly, it is required that the width of the driving pulse of the fuel injection valve per each rotation be set to 2.5 msec or smaller as approximately one fortieth of 100 msec.
Meanwhile, the accuracy of the valve opening time of the electromagnetic fuel injection valve depends on time delay inherent in the mechanical operating parts of the fuel injection valve with respect to the driving electronic signal.
The commonly used fuel injection valve requires about 1.6 msec to move from the closed condition of the valve to the opened condition thereof and about 0.9 msec to move from the opened condition of the valve to the closed condition thereof. Accordingly, as best shown in FIG. 1, illustrating the relationship between the amount of fuel injected and driving pulse width, in the operation of fuel injection in which the width of driving pulse is smaller than about 2 msec, extremely large variations in operating time are apt to occur, thereby making it difficult to maintain the desired accuracy of the amount of fuel supply.
FIG. 2 is an operational diagram showing a fuel injection mode with respect to crank angle at the time of idling of an engine with six cylinders according to the conventional fuel injection system.
As seen from FIG. 2, in an engine with the six cylinders, an amount corresponding to one rotation, i.e. fuel for three cylinders e.g. #1, #5, #3 is injected in a single pulse, while in an engine with four cylinders, a single pulse feeds two cylinders. Assuming that the idling speed is 600 r.p.m., the fuel injection time is only about 2.5 msec. Therefore, with the conventional method in which fuel is injected only once per each rotation of the engine, the fuel is unequally supplied to the cylinders. This results in drawbacks such as polluted exhaust and rough idling.
In order to prevent this problem, another method has been proposed to effect fuel injection in synchronism with the intake stroke of each cylinder.
With this method, with six cylinders the time required for effecting one fuel injection is only 0.8-0.9 msec, while with four cylinder it is 1.2-1.3 msec. Accordingly, in practice it is impossible to supply fuel to each cylinder with a high accuracy.
It is further noted that the distribution of fuel being injected to one cylinder is different from another not only at the time of idling but also at the time of low speed or low load in the range of small intake flow, that is, when the amount of fuel being supplied is small.